Electrical machines contain in the multitude of longitudinal grooves in the laminated stator core, special kinds of coil windings. These coils are generally made of copper, which produce a magnetic field that propagates in all directions by virtue of time-selective applications of current. This field in turn drives the freely rotating rotor suspended in the stator bore, the rotor being able to react to the induced magnetic field in the form of forced rotation, for example, by virtue of a multitude of applied permanent magnets. Thus, electrical energy can be converted to kinetic energy. The laminated stator core is at ground potential, and the coils by contrast at a high kilovolt potential. The coils in the stator grooves must accordingly be electrically insulated with respect to ground.
For this purpose, each and every coil is insulated with a special mica paper-based tape (“mica tape”) multiple times with a defined overlap. Mica is used since it is an inorganic solid insulation material in platelet form that is capable of retarding electrical erosion under electrical partial discharge scenarios for a long periods, and has good chemical and thermal stability. Mica tapes consist, for example, of mica paper and one or more carriers (e.g., foil) joined to one another via a tape adhesive. Mica tapes are used with preference over mica paper, since mica paper alone does not have the mechanical strength necessary for an insulation process.
Accordingly, further additives may be added to the tape adhesive. For example, accelerators may be added that have a catalytic effect on the thermal curing of an impregnating agent subsequently applied externally after the coils insulated with mica tape have been fitted into the laminated stator cores and electrically connected. For avoidance of partial discharges during later operation, the air in the voids of the windings and especially in the groove gaps of the laminated stator core is displaced by the impregnating agent. Since this distance from insulated coil to which current is applied to the laminated core is generally kept as small as possible, field strengths of several kV/mm are by no means rare at that point. The insulation material has to be chosen accordingly.
Thermally curable epoxy resin/anhydride mixtures have been found to be reliable for these purposes. In the “VPI method” known to those skilled in the art as vacuum pressure impregnation, the stators from the electrical machines, composed of their individual parts, together with the fitted and mica tape-insulated coils, are wholly flooded according to the prior art with a mobile epoxy resin/phthalic anhydride formulation in a vacuum chamber and then impregnated under pressure. Depending on the interplay between the accelerator in the mica tape and impregnating agent, there may be gelation of the impregnating agent that has penetrated into the mica tape insulation even during the impregnation phase.
Since phthalic anhydrides, however, are respiratory system-sensitizing substances, there is great interest in the production of entirely anhydride-free insulation systems, such as impregnating resins with oxirane functionalities, which are known, for example, from DE 102014219844.5.
The final curing is generally effected under standard pressure in an industrial kiln. The accelerator in the mica tape (tape accelerator) here has the task of gelation and curing the impregnating resin applied, which has to date always been epoxy resin with phthalic anhydride, within a desired period of time at a defined temperature. The impregnating agent which has become established as the industrial standard is a mixture of distilled bisphenol A diglycidyl ether and methylhexahydro-phthalic anhydride; this very mobile formulation which, in the absence of accelerator substances, has a desirably long storage stability at impregnation temperature (for example doubling of the initial viscosity only after several weeks), but reacts rapidly to give the high polymer in the presence of catalytically active species. Since the mica tape, however, likewise has to have storage stability for a sufficiently long period, the tape adhesive and tape accelerator should be inert with respect to one another. Ideally, all three components (tape adhesive, tape accelerator and impregnating agent) only react with one another at the moment of encounter during the VPI process. This achieves the best possible crosslinking and binding, compatibility and ultimately freedom from faults and cavities and hence long electrical lifetimes of the “main insulation” of the electrical machine that is the ultimate result of the curing.
Since the impregnating agent (“impregnating resin”) has to date always still been an epoxy resin/phthalic anhydride mixture, an amine derivative is often the method of choice for initiation of curing. For instance, the tape accelerator is frequently a substituted amine, for example based on a piperazine or the like, because it is possible by virtue of this species to establish comparatively high glass transition temperatures in epoxy resin/anhydride mixtures on thermal curing. In addition, zinc naphthenate is an established tape accelerator.
Since the tape adhesive is ideally likewise oxirane-functional for optimal compatibility or reaction with the impregnating resin, a problem that arises is that of storage stability in the mica tape. Especially when using anhydride-free impregnating resins, accelerators that initiate anionic and/or cationic polymerization mechanisms are used, and are therefore found to be less inert with respect to the conventional tape adhesives than with respect to an accelerator for an acid anhydride/reactive resin mixture.